Breaking oil tank bracket

Have just installed an early tank with all original non fractured bracketry. Should have realised it would go and it did inside 800 miles. The tank was fitted so I could again repair the rear mount on my original. I have always made sure the tank is not stressed on the rubbers and that everything aligns but it seems I have a peculiar vibration problem that nails the std mount in short order. All pipes are pretty flexible and plenty of movement since that has been mentioned as a problem on another site. Maybe that movement needs to be reduced with some packing around the filler and frame tubes?
Question is: Does anyone else get through rear brackets like I do (every 6-8000 miles with a HD bracket up 50% in guage and width)?
Has anyone come up with a better way than just beefing up the rear mount? I have a feeling if I go too far the stresses will then affect the tank which is pretty thin where the mount is welded.
ISO's are on the upper limit and even slightly over on rear to achieve acceptable vibes at 4000+ (I put up with the very slight "bendy" feeling).
Would appreciate any ideas. Thanks.

Colorado Norton Works has a fix for that. Haven't done it myself yet, but probably will at some point. - BrianK

http://www.coloradonortonworks.com/catalog/mods.asp

The CNW fix has been around for a while and addresses the stress on the bottom mount, not the rear. That stress is usually caused when the stock Norton front or rear mount separates or the rear strap breaks and lets the tank bounce on the bottom mount.

You say you are using the HD bracket, I assume you mean the rubber buffer mount from a shovelhead oil tank?

I have three Commando's running regularly, and have repaired two rear brackets with slightly thicker steel strap brazed to the tank and the HD mounts front and rear. I have not had a failure since installing them some 10 years ago. Make sure your strap that you brazed or welded onto the tank is bent such that it is not stressed when you bolt it up.

My brackets broke right at the edge of the weld on the tank. Is that where you are seeing failure?

By the way, I haven't tried eliminating the bottom mount, but it sounds like a reasonable idea if you are already welding on the tank.

Thanks gents. Yes, it is the rear bracket I termed HD meaning heavy duty, not Harley. On the original tank I increased width to 1" and guage to 090". Latest breakage is a stock tank bracket right between the welds as Ron says. The "new" tank I restored had all original brackets and no damage or repairs other than a rust hole adjacent the banjo feed. I figured if it had survived many years in another Commando it must be good for a few more!
I had beefed up the base by epoxying a 1/8" aluminum plate as tight as I could infilling with adhesive under the boss. In 17k miles this has not shown any cracks. Did the same on replacement tank. Probably not as good as brazing/welding but so far OK. Don't have any problems with the stock mounts. Are the Harley parts stiffer or just resistant to tearing the rubber?
Good point about the bracket bend but I'm sure there was no real stress here, with tension between the two mounts you mean?
Any idea on which plane of vibration is doing the most damage? I'm guessing it's the fore and aft plane or am I way off?
The CNW modification includes a high density pad and looks quite thick. Would you consider that helpful as I'm thinking it might stress the lower mount more if tank to battery tray clearance is tight?
I'd like to keep the stock rear bracket but looking at the mounting another way I thought maybe a tab bent and welded to the tank seam, right on the radius would better resist the vibes but then again there would be a 90 deg bend and twist in the steel, perfect for breakage unless it was very heavy guage.
Thanks for the advice.

Keith1069,

I believe much of the failure is due to the rear bracket "flexing" with vibration. The thicker Harley Davidson rubber mounts tend to hold the tank more steady. The original thin mounts allow the tank to bounce around much more. I think this is why the tank lasts longer with these mounts. YMMV.

Interesting, I've found that loosening the isos helps reduce vibration and thus prevents fatigue failure of the oil tank bracket.

I woud like to install the Harley rubber gizmo as a safeguard; does anyone have the part number?

I don't have a part number, but this is what you want
http://cgi.ebay.com/ebaymotors/twin...1133087QQihZ001QQcategoryZ10064QQcmdZViewItem

Somebody usually has a bin of them at the local Harley swap meets.

I usually shorten the studs before mounting to make it easier.

Thanks Ron.

Appreciate that Ron, will look around for those mounts. In meantime I decided a brace from the bracket to the filler neck will help stop the flexing. Maybe two, one from each side as triangulation.

Well I took two of the Commando muffler mount rubbers and after enlarging the oil tank bracket mount holes the rubbers can be fitted at a tight squeeze .. after all these are substantially larger and robust then the original RUBBISH!!!

I retained the bottom mount a basic support only and have had no problems since !